In printed circuit board fabrication, SLC circuit layers and circuit layers within the conventional printed circuit board are interconnected by through holes which have been drilled and plated. Due to drilling and plating limitations, these plated through holes are quite large with respect to the SLC wiring, and remove valuable wire routing real estate from the SLC layers.
One method of filling holes involves forcing a fill composition, typically a copper filled epoxy composition, through a mask into the holes of the printed circuit board. However, the fill material can bleed between the mask and/or leave nubs at the surface of the filled through hole. This fill process further requires a polishing step after to remove the nubs and the bleed from the surface. The polishing process itself is not perfect. Often times, a small nub will be left after polishing or the fill material may tear out of the hole, leaving a depression at the center of the hole. Both residual nubs and depressions can cause problems during subsequent circuitization steps. The technique of forcing a fill composition through a mask also requires expensive paste, and requires the step of drilling the mask.
Other techniques for filling of holes involve screen printing a liquid epoxy material into the holes, or covering the holes with a dry film. Both of these techniques tend to trap a small air pocket at the center of the hole which can lead to cracking of the overlying circuit lines during thermal cycling.
The filling of vias also encounters similar problems. The screening or tenting processes can cause cracks in the encapsulant materials during thermal cycling. These cracks in the encapsulant will allow moisture penetration to the device, and can also propagate through the encapsulant and cause chip cracking.
It would be desirable to have an inexpensive and efficient technique for reliably and completely filling holes that does not trap air in the through hole, cause bleeding, or leave nubs or depressions at the surface of the filled through holes.